European Galactic Plane Surveys (EGAPS), Uncovering the biggest secrets in our Galaxy

We have only touched the tip of the iceberg with EGAPS explorations.

In this article, I present the current status of the EGAPS surveys, and facing challenges.

The European Galactic Plane Surveys (EGAPS) are a landmark. With EGAPS, one billion star systems will be mapped, covering the entire Galactic Plane, with unique depth and precision, enabling scientists to respond to labyrinthine questions that are baffling to the formation, structure, and evolutionary history of our Galaxy.

The EGAPS project is open and participatory. The main actors are Radboud University (Nijmegen, The Netherlands) together with the University of Cambridge, the University of Hertfordshire, and the University of Graz. In support of the EGAPS scientific goals, besides maximizing its impact on society, affiliated world-class institutions around the globe collaborate.

see RU web page: https://www.astro.ru.nl/wiki/research/surveys

EGAPS surveys are fourfold: The so-called Northern Milky Way surveys (IPHAS and UVEX ), coupled with the VPHAS+ and OmegaWhite, which survey the Southern Milky Way and Bulge. The capability of these three surveys and how they harmonize to other Plane surveys are described by Barentsen et al. (2015).

I have divided this article into five sections. In the first section, I would like to draw your attention to the white dwarf stars, before looking each one of the EGAPS surveys (second, third, and fourth sections). In the path of science, it is not all a bed of roses; I address nagging worries in the fifth and final section.


1. Bewitching smile to white dwarf stars

Variable binaries with hot blue-white accretion disks around a hardly visible white dwarf (left, middle) or black hole (right). Credits: Mark. A. Garlick.

EGAPS is second to none in the search for interaction with white dwarfs.

The populations of white dwarfs play a crucial role in our understanding of not only our Galaxy but also the universe. White dwarfs constitute end points of 97 per cent of stars in the Milky Way. Accreting white dwarfs trigger explosions (e.g. supernovae) which provides a seed-bed for the formation of planets and stars.

The most common class are the called Cataclysmic variables (CVs), in which white dwarfs accrete matter from typically core-hydrogen-burning main sequence stars, usually via an accretion disc that is subject to thermal instabilities that trigger luminosity outbursts.

The AM CVn stars are a subtype of cataclysmic variable, in which a white dwarf is accreting hydrogen-depleted matter. Unlike typical CVs, companions in AM CVn stars are helium-core white dwarfs, non-degenerate helium-core-burning stars, or terminal-age main sequence stars. AM CVns are also characterized for having orbital periods shorter than one hour. They are thus also referred to as ultra-compact binaries.

Why AM CVn stars are of astrophysical significance? Scientists predict AM CVs as the strongest source of gravitational waves, which are expected to be detected, in the future, by the Evolved Laser Interferometer Space Antenna (eLISA). Last but not least, their space density serves for testing and calibrating Galactic models of binary stellar populations.


2. IPHAS - The INT Photometric H-alpha Survey of the Northern

See photo gallery: http://www.iphas.org/gallery.shtml

Starting observations in 2003, now the IPHAS has surveyed about 2000 square degrees of the Milky Way’s North plane (in latitude range -5° < b < 5°), deploying the 2.5m Isaac Newton Telescope (INT), in the Canary Islands, Spain.

What is outstanding with IPHAS is the Wide Field Camera on the INT telescope, which resolves 0.33 arcsec per pixel, coupled with registers of H-alpha magnitudes, to recognize transitory non-equilibrium events that shape the stellar life cycle of stars.

IPHAS drives science forward. In 2014, the survey came to its second edition, IPHAS DR2, charting 219 million individual stars, listed with near one hundred attribute. The sample contains new supernova remnants, cataclysmic variables (CVs), and sub-types. See also the press release from the Royal Astronomical Society, for further information.

Now, with improvements in data technologies, scattering data hurdles aggravated by photometric calibration errors or stars in differing fields have been cleared.

IPHAS has become a landmark to bring out H-alpha emitting star systems. In 2006, Witham et al. evaluated how IPHAS recovers CVs on basis the H-alpha emission lines, and found a recovery rate of 70 per cent. Additionally, it was demonstrated that photometric H-alpha measurements fix to CV data of biases against detection of intrinsically faint, short-period systems.

Not only does IPHAS stand out for being a photometric survey in H-alpha, but it is also the first digital proper motion survey of the Galactic Plane.

In addition to a narrow-band H-alpha, IPHAS filters in two redder broad-band magnitudes (r, i) of the Sloan Digital Sky Survey. We shall learn the increase of luminosity of those stars brighter than about 20th magnitude. These capabilities enable the study of distant clusters.

As impelled, in 2014, Sale et al. presented a high spatial resolution three-dimensional map of dust extinction across the northern Galactic Plane.

A density map of part of the Milky Way disk, constructed from IPHAS data. The Most Detailed Catalogue Ever Made of the Visible Milky Way see http://www.ing.iac.es/PR/press/iphasMW.html

Among recent discoveries, characterized by their H-alpha emission, there are many rare objects, such as OB stars and OBA extreme super-giants. Highly evolved lower-mass red giants (post-AGB stars). Be stars of all types. A range of interacting binary stars: symbiotic stars, super-soft compact binaries, and WD/NS/BH accreting binaries generally. Also, luminous blue variables (P Cygni and eta Car like objects), Extended planetary nebulae, and T Tau stars.

In summary, it is undeniable that IPHAS has extended the scope of data analysis –and it will continue, compared to the legacy of earlier wide-area surveys. Currently, IPHAS holds more than 70 publications (see http://www.iphas.org/publications.shtml).


3. UVEX -The UV-Excess Survey of the Northern Galactic Plane

Close-up of the Rosette Nebula. Light-years long dust filaments are silhouetted by luminous hydrogen gas in this star-forming region in the Milky Way at 5000 light-years. (Credits: Nick Wright)

The UVEX survey also uses the INT/WFC Telescope to image a 10185 wide band, centred in the Galactic equator.

Observations started in 2006, tailoring to the UV-excess populations: narrow-band He I 5875, RGO U-band filter, and broadband magnitudes (g,r) of the Sloan Digital Sky Survey.

The UVEX is able to reach stars down to a limiting magnitude of 21–22 in the g-band, which is perfect for shaping the structure of our Galaxy.

Furthermore, UVEX also provides a second epoch r-band for a deep high precision proper motion survey of the Galactic plane.

Accomplished 30 per cent of the survey coverage, Radboud University (RU) reported, in 2009, the first-ever homogeneous blue survey of the Galactic Plane. Besides that, the report establishes the observational procedure (e.g. setup, design, and data processing), and the scope of the scientific goals.

The available UVEX catalogue classifies more than 10,000 stars, with high quality proper motions, being the essential part white dwarfs of hydrogen and helium atmospheres, CVs and AMCVn stars, sub-dwarfs B stars, metal deficient stars, and quasars behind the galactic plane.

One of the scientific aims of UVEX is to find the space density and white dwarfs, which is a need for the validation of stellar models. In this sense, recently RU has contrived to it (see their study, for more enlightenment).


4. VPHAS+ and OmegaWhite: Surveys of the Southern Galactic Plane and Bulge

The VST Photometric H-alpha Survey of the Southern Galactic Plane and Bulge (VPHAS+) is a public survey project of the European Southern Observatory (ESO), and uses the VLT Survey Telescope in Chile (VST).

a VPHAS-based mosaic of the star-forming region Messier 8 (M8, or the Lagoon Nebula). See more at http://www.vphasplus.org/gallery.shtml

VPHAS+ has been surveying since 2012, through five magnitude filters (u, g, r, i, and H-alpha), and barely a quarter of the southern sky and bulge has been catalogued (VPHAS-DR2).

Preferred populations passing through the amalgam of filters are A-type stars, post-common-envelope binary stars, red giants, and dwarfs.

In addition to bolstering the survey observations and strategies of its northern twin survey, VPHAS+ forges finer quality of the image, and ameliorates statistics.

VPHAS+ is susceptible to reach 500 million star systems, up to an order of 22.5th magnitudes in the g-band, and collect stars with an impeccable accuracy of up to 0.02 magnitudes.

This is so because mounted to VST is the OmegaCam, an imaging camera coupled with 32 charge-coupled devices (CCD), which result in 256 million pixels!

The sensitivity of OmegaCam is such that it detects many of the reddened stellar populations of the Galactic Plane.

Greater than ever, O-B-A stars will be monitored, by a reason of 10, dissimilar to previous surveys.

Not only will VPHAS+ help astronomers to sharpen the star formation history of our Galaxy, but it will also conform the structure of the disk and extinction map of the southern hemisphere, in unprecedented detail.

Specifically, the presence of the u-band will bring about far-reaching implications on subject areas, such as central stars of planetary nebulae, OB stars, and interactive binaries with a dominant hot star, as well as searching for white dwarfs, hot subdwarfs, and horizontal branch stars.

Let me now shift the focus to the OmegaWhite survey, considered a distinct unit of EGAPS.

OmegaWhite is surveying repeatedly 400 square degrees, intersects regions of observations with VPHAS+, and operates with the photometric systems of the OmegaCam on the VST.

In contrast, the OmegaWhite is exclusively picking up transitory and variable star systems, applies a short term photometric variability scheme, and adds a narrow band filter in He I 5015

Radboud University (RU) has played a major role in OmegaWhite, by developing the instrumentation (X-Shooter, a new high efficiency, wide-band spectrograph), conducting the observations, as well as performing data reduction, processing, and analysis.

In the first study of the OmegaWhite, recently published, RU identified more than 60,000 stars systems, corresponding to observations that took place throughout December 2011 and April 2015, with coverage of only 26 square degrees.

OmegaWhite thrives in areas where tangible progress is needed. Findings confirm OmegaWhite as the swiftest and surest route to determining the closest white dwarf binaries (with orbital periods < 80 min), as never before.

Moreover, OmegaWhite constitutes the vanguard of research in AM CVn stars, which are an absolutely scientific mystery, not to mention the fact that the survey serves as baseline to assess stellar pulsations/variations.


Nagging Worries

Scientists struggle to face biases and selection effects challenges on their way to strengthening stellar models. I bet you would agree if I dared to say that nothing is worse for the astronomer than questioning their work on the grounds of factual accuracy.

Any theory should be validated on unbiased observations and evaluation procedures, but to date, this is awkwardness.

It can be contradictory that in the era of the largest surveys, the selection effects may show untruthful traits of a stellar population; however, my experience says it is too common.

What should demand our attention? Science goes wrong when misinterpretation arises from instrumental inaccuracy, statistically significant data omitted, and inefficient ways to contextualize and analyze data.

To make matter worse, obstacles during the reduction and processing of the data might be tormenting for scientists.

For example, crucial problems that arise from image processing are bad columns, traps, dust on filters, cosmic rays, saturation trail, diffraction, and trailing. See the ESO Webpage for a clear description of them.

To sum up, it is compelling evaluating atmospheric effects (e.g., refraction, twinkling, air mass, extinction, seeing), quality image (point spread function — PSF — aberrations), resolution (spatial, temporal, and spectral), and any source of noise (e.g., sky, background, thermal, readout).

What is more, recently, the International Astronomical Union has urged immediate action to tackle assumptions in stellar models, especially is phases when stars are out of thermal equilibrium.

Data prone to misclassification are, for instance, unresolved star systems appearing as single-line spectra or without lines.

Furthermore, systemic incongruities may occur when fitting pattern spectra — uncertainty increases with the coolest stars, when no radial velocities or failures are exhibited as a result of instrumental sensitivity.

The orbital period of a binary star acts a filter. The binary stars with the shortest orbital periods will likely weigh more in the sample size than their counterparts of larger periods.

On top of that, the largest orbital-period systems will likely show no radial — one of the distinguishing features that scientists use to identify binary stars. As a result, they might be confused with single stars.

Lastly, the duration of the observing time limits the orbital period to a maximum and minimum permissible value; consequently, a portion of the binaries will be not observed.

Supposing you are an earlier astronomer, in my thesis you can learn more about issues of biases and selection effects in observational astronomy.

How much we should trust EGAPS observations?

Not only has RU critically evaluated some sample biases and selection effects associated with EGAPS data, but also they have tackled them.

RU has devoted full strength to address the UVEX’s critical errors, by determining: 1) The cumulative frequency of seeing to assess how the atmosphere conditions of Canary Island influences the UVEX observations. 2) The crowding correction to solve the brightness in overlapping stars. 3) The error in the stellar magnitude in the four different filters (U, g,r, He I 5875). 4) Effects of reddening for white dwarfs. 5) Selection effects due to photometric calibration.

When it comes to the OmegaWhite Data, as illustrated examples of them, are the following:

First, by carrying out observations irregularly, researchers of RU have prevented aliasing that provokes a blurring of the image.

Secondly, they have withstood systematic tilts in the measured light curves of the stars, which are triggered by atmospheric extinction, by applying a seemly algorithm.

Thirdly, they concluded that approximately 20% of the OmegaWhite data is under par photometry; on account of bad columns, low signal-to-noise ratio (the faintest stars), saturation trail (the brightest stars).

Lastly, stellar crowding introduces magnitude errors, resulting their methodological approach having consisted in setting a limit in brighter magnitude and signal-to-noise ratio, on the reference stars, and building a customised index.


More Publications from RU researchers

The European Galactic Plane Surveys: EGAPS. Exploiting Large Surveys for Galactic Astronomy, 26th meeting of the IAU, Joint Discussion 13, 22–23 August 2006, Prague, Czech Republic, JD13, #54

The IPHAS-POSS-I proper motion survey of the Galactic plane. Monthly Notices of the Royal Astronomical Society, Volume 397, Issue 3, pp. 1685–1694.

Mapping the Milky Way in visible light: the IPHAS, UVEX and VPHAS+ surveys. IAU General Assembly, Meeting #29, id.2257508.

A determination of the space density and birth rate of hydrogen-line (DA) white dwarfs in the Galactic plane, based on the UVEX survey. Monthly Notices of the Royal Astronomical Society, Volume 434, Issue 4, p.2727–2741.

VizieR Online Data Catalog: UVEX sources spectroscopic follow-up (Verbeek+, 2012). VizieR On-line Data Catalog: J/MNRAS/426/1235. Originally published in: 2012MNRAS.426.1235V.

Spectroscopic follow-up of ultraviolet-excess objects selected from the UVEX survey. Monthly Notices of the Royal Astronomical Society, Volume 426, Issue 2, pp. 1235–1261.

A first catalogue of automatically selected ultraviolet-excess sources from the UVEX survey. Monthly Notices of the Royal Astronomical Society, Volume 420, Issue 2, pp. 1115–1134.

The UV-Excess survey of the northern Galactic plane. Monthly Notices of the Royal Astronomical Society, Volume 399, Issue 1, pp. 323–339.

The OmegaWhite Survey for short period variable stars II: An overview of results from the first four years. Accepted for publication in MNRAS.

UVES and X-Shooter spectroscopy of the emission line AM CVn systems GP Com and V396 Hya. Monthly Notices of the Royal Astronomical Society, Volume 457, Issue 2, p.1828–1841

Galactic Binaries with eLISA. The 9th LISA Symposium, Proceedings of the conference held 21–25 May 2012 at Bibliothéque Nationale de France, Paris. ASP Conference Series, Vol. 467. San Francisco: Astronomical Society of the Pacific, 2013., p.27

Spectroscopic follow-up of ultraviolet-excess objects selected from the UVEX survey. Monthly Notices of the Royal Astronomical Society, Volume 426, Issue 2, pp. 1235–1261.

A determination of the space density and birth rate of hydrogen-line (DA) white dwarfs in the Galactic plane, based on the UVEX survey. Monthly Notices of the Royal Astronomical Society, Volume 434, Issue 4, p.2727–2741.

Nova Report 2006–2007.

Access to Astronomical Catalogues.

More publications in the The SAO/NASA Astrophysics Data System (ADS).

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